C++ class 初始化赋值问题？

Question

第一个有自动生成默认构造函数吗？
为什么第二个不可以编译通过？

Answer 1

Does the first one automatically generate a default constructor?

Yes, when there is no any user-defined constructor in a class, the compiler will generate a default constructor. See Default constructors.

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Why doesn’t the second one compile?

If a class wants to use a brace list to initialize members, then the class must meet the conditions of aggregate class. Among the conditions of aggregate class, one is

The

class does not contain any in-class initializer.

In the second example class definition of the subject, both members a and b use in-class initializer, so they are not aggregate class, so

cannot be used

NoDefault y = {2, 2};

Initialized using

.

Note: According to aggregate initialization, this condition will be deleted in C++17.